Low molecular protease inhibitors originating from natural sources such as leupeptin, chymostatin and elastatinal are known. Based on these compounds, a number of peptidyl aldehydes have been synthesized and provided as inhibitors. It is known that upon inhibition of serine protease or thiol protease, these peptidyl aldehydes are known to form a covalent bond to the hydroxy or thiol group of the enzyme, see Thompson, R. C., Biochemistry, 12, 47-51 (1973).
Peptidyl aldehydes contain an aldehyde group at the C terminus of peptide chain. Then, modification of their amino acid sequences to enhance the specificity or inhibitory activity is restricted to the N-terminal site. Compounds including Poststatin, that were found by the present inventors, contain also at the C terminus as the activity center such a structure that enhances the specificity and contain an .alpha.-keto acid moiety in peptide chain; it is thus considered that these compounds would inhibit the enzyme in the same mechanism as in the peptidyl aldehydes, cf. EP-A-0423358, EP-A-0468339 and U.S. Pat. No. 5,221,752. However, poststatin and the like might be degraded by various proteases in vivo since they have the peptide residue and amino acid residue bound to the a-keto acid structure at the C and N termini. It is therefore expected to develop compounds having other stable functional groups with less possibility of degradation.